In recent years, there has been an increasing demand for a reduction in the size of a compressor for use in the refrigerating cycle. This is achieved by employing a rotary type compressor in place of a reciprocating type compressor.
However, the rotary compressor has a drawback in that the motion of a roller is unstable because the direction of the rotation thereof on its own axis changes during a single rotation thereof, deteriorating the volumeric efficiency thereof.
A conventional rotary compressor will be described below in detail with reference to FIGS. 1 through 4.
Reference numeral 1 denotes a sealed casing and 2 denotes an electric motor portion which is coupled, through a shaft 3, to a mechanical portion body 9 including a cylinder 4, a roller 5, a vane 6, a main bearing 7 and a sub bearing 8. The shaft 3 has a main shaft 3a, a sub shaft 3b and a crank 3c which is eccentric from the axis of the main and sub shafts 3a and 3b by E. The shaft 3 has a hole 3e at the center thereof, and the crank 3c has an oil supplying hole 3f and an oil supplying groove 3g. Reference numeral 10 denotes a spring provided on the rear surface of the vane, and 11a and 11b respectively denote a suction chamber and a compression chamber formed within the cylinder 4 by the roller 5, the vane 6 and the main and sub bearings 7 and 8. The inner peripheral sides of end surfaces 5a and 5b of the roller 5 which respectively face the main and sub bearings 7 and 8 are tapered to form tapered portions 5c and 5d whose cross-sectional area decreases toward the outer peripheral side thereof. Reference numeral 12 denotes an oil supplying mechanism coupled to the shaft 3. Reference numeral 13 denotes a suction pipe which communicates with the suction chamber 11a via a suction passage 14 formed in the sub bearing 8 and the cylinder 4. 15 denotes a discharge hole which communicates with the interior of the sealed casing via a discharge valve 16. 17 denotes a discharge pipe which is opened into the sealed casing 1. 18 denotes a lubricating oil.
In FIG. 4, the arrow of the solid line indicates the direction of the motion of the roller 5 which is obtained at a certain time during the operation of the compressor, and the arrow of the broken line indicates the direction in which the lubricating oil 18 flows over the end surfaces 5a and 5b of the roller as a consequence of the operation of the roller 5. Reference numeral 5e denotes a portion of the tapered portion 5c or 5d of the roller 5 whose cross-sectional area gradually decreases in the direction indicated by the arrow of the broken line, and 5f denotes a portion whose cross-sectional area gradually increases in the same direction.
The compression mechanism of the rotary compressor will now be described. A refrigerant gas supplied from a cooling system (not shown) passes through the suction pipe 13 and the suction hole 14, and then reaches the suction chamber 11a of the cylinder 4. Thereafter, the refrigerant gas is gradually compressed by the rotary motion of the shaft 3 which is generated by the rotation of the electric motor portion 2 in the compression chamber 11b defined by the roller 5 rotatably supported by the crank 3c of the shaft 3 and the vane 6. The compressed refrigerant gas is discharged into the interior of the sealed casing 1 through the discharge hole 15 and the discharge valve 16, and then discharged into the cooling system through the discharge pipe 17.
The high-pressure lubricating oil 18 with the refrigerant contained therein and contained in the sealed casing 1 is supplied to the hole 3e of the shaft 3 by means of the oil supplying mechanism 12. Thereafter, the lubricating oil 18 is supplied to the sliding portion of the main and sub bearings 7 and 8 and to the crank 3c and the inner peripheral side of the roller 5 from the oil supplying hole 3f and the oil supplying groove 3g to lubricate the roller end surfaces 5a and 5b. Subsequently, the lubricating oil 18 passes through the suction chamber 11a and the compression chamber 11b, is discharged into the sealed casing 1 from the discharge hole 15, and then stays at the bottom of the sealed casing 1.
As the shaft 3 is rotated, the roller 5 rotates while turning round about the crank 3c in either of two directions. Consequently, the locus of a certain point on the roller 5 is spiral, and the direction of the movement of the roller 5 changes about 360.degree. while the shaft 3 is rotated. Assuming that the direction of the spiral motion of the roller 5 is that indicated by the arrow in FIG. 4, since the tapered portions 5c and 5d are provided on the end surfaces 5a and 5b of the roller 5 and the cross-section of the portion 5e gradually decreases toward the outer diameter side of the roller, only the lubricating oil 18 which flows into the vicinity of the portion 5e in the tapered portion 5c or 5e generates an oil pressure due to the wedge effect. Consequently, the oil pressure near the tapered portion 5c balances the oil pressure near the tapered portion 5d, and the roller 5 is thus retained such that a clearance .delta.a between the roller 5 and the main bearing 7 is equal to a clearance .delta.b between the roller 5 and the sub bearing 8. The amount of lubricating oil with the refrigerant contained therein which flows into the suction chamber 11a and the compression chamber 11b from the crank 3c through the roller end surfaces 5a and 5b is proportional to the cube of the clearance. Therefore, where .delta.a+.delta.b=constant, the amount of lubricating oil which flows in is at a minimum when .delta.a=.delta.b. Thus, the provision of the tapered portions 5c and 5d assures a compressor exhibiting an excellent volumeric efficiency and hence a high efficiency.
Such a compressor is disclosed in, for example, Japanese Utility Model Publication No. 61-20317.
However, in a compressor which has the above-described structure and in which the thickness of the roller, indicated by (outer diameter - inner diameter)/2, is small and the ratio of the high pressure to the low pressure during operation (compression ratio) is high, as in the case of a small compressor for refrigeration having a small cubic capacity, even if the clearance between the end surface of the roller and the main bearing is made equal to the clearance between the end surface of the roller and the sub bearing by the provision of the tapered portions, the clearance of the tapered portion provided over the entire periphery practically increases by a value corresponding to the amount of taper, and the sealed distance of the flat surface where no tapered portion is provided decreases over the entire periphery, increasing the amount of lubricating oil with the refrigerant contained therein which flows into the suction chamber and the compression chamber. Thus, the provision of the tapered portion does not ensure reduction in the leakage loss and improvement in the volumeric efficiency.
The above-described structure utilizes the wedge effect of the lubricating oil which enters the tapered portion. This wedge effect is generated by the component of the roller rotation in the spiral motion thereof caused by the rotation of the shaft and not generated by the component of the roller rotation about the crank, because the cross-sectional area of the tapered portion remains the same in the circumferential direction, and is thus low. Furthermore, the oil pressure is generated by the wedge effect only at the single portion on the end surface of the roller, and no oil pressure is generated at most of the portion of the end surface. Furthermore, since the tapered portion has a shape which continues in the circumferential direction, the pressure generated by the wedge effect may escape in the circumferential direction, reducing the pressure generated by the wedge effect. Therefore, the stability of the roller achieved by the wedge effect is not sufficient, and the improvement in the volumeric efficiency is low.